Interface between a noninvasive blood pressure sensor and an invasive blood pressure monitor

ABSTRACT

The preferred embodiment of the present invention comprises a single microprocessor-based interface that connects between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor or module. The interface effectively emulates an IBP transducer in such a way that the IBP monitor sees the interface as if it were a regular IBP transducer from a fluid-filled blood pressure monitoring line. It receives the signal from an NIBP sensor and determines the blood pressure corresponding to the signal. It accepts the excitation voltage provided by the IBP monitor. From the excitation voltage and a known transducer sensitivity which the IBP monitor is configured to work with, the interface emulates the IBP transducer output signal corresponding to the blood pressure. The interface also emulates the input and output impedances of the IBP transducer which the IBP monitor is configured to work with. Zeroing of the interface with the IBP monitor can be easily performed in a way that is similar to that for a fluid-filled system. A noninvasive system comprising a suitable NIBP sensor and this interface can be used as an alternative to the fluid-filled monitoring line.

This application is a divisional of, and claims the benefit under 35U.S.C. §121 of U.S. patent application Ser. No. 10/376,655, filed onFeb. 27, 2003 now U.S. Pat. No. 7,144,372.

FIELD OF INVENTION

The present invention relates to blood pressure monitoring, particularlybut not solely to an interface between a noninvasive blood pressuresensor and an invasive blood pressure monitor.

BACKGROUND ART

Blood pressure is one of the most important vital signs used in theassessment of a patient's cardiovascular health. In critical care, it isusually monitored continuously using an invasive fluid-filled monitoringline, also called an arterial line, in which a catheter is inserted intoan artery and blood pressure from the artery is transmitted to a bloodpressure transducer through fluid-filled tubing 12 to 84 inches long.The arterial pressure as measured by the transducer is displayed on aninvasive blood pressure (IBP) monitor. A schematic diagram of such asystem is depicted in FIG. 1.

This intra-arterial method not only allows arterial pressure to bemonitored continuously on a beat-to-beat basis, but also allows arterialblood to be sampled through the fluid-filled system without the need tocannulate another arterial site. In many hospitals, IBP monitors formpart of a central monitoring system in which arterial pressuremeasurements from patients at various locations in the hospital can bemonitored from a central location or from other locations in thehospital. However, this invasive method of monitoring blood pressure isassociated with risks of complications such as infection, thrombosis andair embolism.

Noninvasive measurement methods that provide continuous beat-to-beatblood pressure offers an alternative to invasive blood pressuremonitoring because they do not carry with them the risk of complicationsassociated with invasive monitoring. The arterial tonometry and vascularunloading methods are two such methods. These methods can be used tomeasure blood pressure in situations that do not justify the use ofinvasive means, especially for patients who do not already have anarterial line in place and who also do not require arterial bloodsampling.

Commercial noninvasive blood pressure (NIBP) monitors that providecontinuous beat-to-beat measurement are mostly standalone monitors thatnot only cannot or cannot be easily connected to a central monitoringsystem, but also require a separate monitor to display their waveforms.For example, the Model 7000/CBM-7000 NIBP monitor and the Pilot/BP-508multiparameter monitor by Colin (Komaki, Japan), both of which providetonometric blood pressure measurement at the radial artery, arestandalone monitors with their own displays and cannot be easilyconnected to a central monitoring system. The same applies to theFinapres® 2300 NIBP monitor and 2350 NIBP/SpO₂ monitors by Ohmeda (nowDatex-Ohmeda, Madison, Wis., U.S.A.) and the USM-803 NIBP monitor byUEDA Electronic Works (Tokyo, Japan), all of which measure continuousbeat-to-beat blood pressure at a finger using the vascular unloadingmethod. A block diagram showing the main elements of such monitors ispresented in FIG. 2.

Another commercial NIBP monitor, the Vasotrac® APM 205A by Medwave(Minneapolis, Minn., U.S.A.), measures blood pressure continually byproviding one beat of the pressure waveform for approximately every 15heartbeats, along with the corresponding systolic, diastolic and meanarterial pressure readings. It uses a modified oscillometric method inwhich various cycles of increasing and decreasing pressure are appliedto the radial artery over a period of 15 heartbeats, and blood pressureis derived from the characteristics of the pressure signal detected bythe sensor over this period of time. The main elements of this monitorare the same as those in FIG. 2.

The Vasotrac APM 205A is a standalone monitor. However, the companymarkets an optional interface, called the NIA V-Line, which connects theVasotrac APM 205A to an existing IBP monitor to enable the pressurewaveform to be displayed on the IBP monitor. A block diagram of anapplication of this interface is presented in FIG. 3. This interface isassociated with U.S. Pat. No. 6,471,646 entitled ARTERIAL LINE EMULATOR.One drawback of this interface is that it requires the use of theVasotrac itself in order for it to work, so a hospital that only wishesto display the NIBP waveform from the Vasotrac on its existing IBPmonitors must purchase the Vasotrac in addition to the interface. Thissituation adds to the procurement costs for the hospital.

SUMMARY OF INVENTION

It is an object of the present invention to provide a device that goessome way toward overcoming the above disadvantages, or which will atleast provide the public with a useful choice.

In a first aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor;

wherein said first input is connected to an NIBP sensor and said secondinput and said output are both connected to an IBP monitor.

Preferably said second input and said output are configured to connectto an IBP monitor by means of a detachable or configurable connectionbetween said interface and an IBP interface cable for an IBP monitor.

Preferably said connection includes an adapter with a first connectorconfigured to connect to said interface and a second connectorconfigured to connect to the transducer end of an IBP interface cable.

Preferably said adapter includes a cable between said first and secondconnectors.

Preferably said adapter does not include a cable between said first andsecond connectors.

In a second aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor;

wherein said processor(s) is configured such that a user can select froma predetermined range of processing options.

Preferably said processor(s) comprises an embedded microprocessorsystem, a personal computer, or a combination of the two.

Preferably said processing options include an embedded microprocessorsystem, a personal computer, and a combination of the two.

Preferably said combination is configured such that said embeddedmicroprocessor system and said personal computer work independently suchthat said interface can receive said measurement signal and saidexcitation signal and can generate an output signal indicative of theIBP of a subject according to predetermined instructions without saidpersonal computer being used, and said personal computer forms anoptional part of said interface and is used for additional signal anddata processing.

Preferably said combination is configured such that both said embeddedmicroprocessor system and said personal computer cooperate to receivesaid measurement signal and said excitation signal and to generate anoutput signal indicative of the IBP of a subject according topredetermined instructions.

In a third aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor;

wherein said processor(s) is configured such that a user can select thesensitivity of said output signal in relation to said measurement signalfrom a predetermined range of choices.

Preferably said sensitivity can be selected or specified by a user.

Preferably said range includes 5 μV/V/mmHg and 40 μV/V/mmHg.

In a fourth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor; configured to determine the differentialvoltage of said excitation signal; and configured to determine themidpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signaland said differential voltage and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to receive said midpoint voltage and configured toprovide said output signal in a form suitable for input to an IBPmonitor; and such that the midpoint of said output signal issubstantially similar to that of said midpoint voltage.

Preferably said second input includes a voltage divider, a differentialamplifier, or a combination of the two configured to sense and conditionsaid differential voltage.

Preferably said processor(s) is configured to receive both positive andnegative ranges of said conditioned differential voltage through acircuit that includes an analog-to-digital converter (ADC).

Preferably said analog-to-digital converter (ADC) is a bipolar ADC.

Preferably said second input includes a voltage divider or a combinationof a voltage divider and a differential amplifier, configured to senseand condition said midpoint voltage.

In a fifth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor; configured to determine the differentialvoltage of said excitation signal; and configured to determine themidpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signal,said differential voltage and said midpoint voltage and emulate anoutput signal indicative of the IBP of a subject according topredetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and such that the midpoint of said outputsignal is substantially similar to that of said midpoint voltage.

Preferably said second input includes a voltage divider, a differentialamplifier, or a combination of the two, configured to sense andcondition said differential voltage.

Preferably said processor(s) is configured to receive both positive andnegative ranges of said conditioned differential voltage through acircuit that includes an analog-to-digital converter (ADC).

Preferably said analog-to-digital converter (ADC) is a bipolar ADC.

Preferably said second input includes a voltage divider or a combinationof a voltage divider and a differential amplifier, configured to senseand condition said midpoint voltage.

Preferably said processor(s) is configured to receive both positive andnegative ranges of said conditioned midpoint voltage through a circuitthat includes an analog-to-digital converter (ADC).

Preferably said analog-to-digital converter (ADC) is a bipolar ADC.

Preferably said processor(s) includes a bipolar digital-to-analogconverter (DAC) to provide said midpoint voltage to said output.

Preferably said output includes a voltage divider or a combination of avoltage divider and a differential amplifier, configured to scale andcondition the output signal of said digital-to-analog converter (DAC)such that the midpoint voltage of the scaled and conditioned signal issubstantially similar to said midpoint voltage.

In a sixth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and wherethe differential voltage of said output signal depends on saidexcitation signal, a predetermined or selectable transducer sensitivity,and said measurement signal; and

an output configured to provide said output signal in a form suitablefor input to said IBP monitors, including a digital-to-analog converter(DAC) to receive output signal from said processor(s), and said DAC isconfigured according to predetermined instructions such that itsfull-scale output voltage range includes the voltage range correspondingto a predetermined maximum pressure range of the NIBP of a subject.

Preferably said digital-to-analog converter (DAC) is a configurablebipolar DAC;

said processor(s) is further configured to configure said DAC; and

said DAC is further configured to optimize the resolution of saidfull-scale output voltage range.

Preferably said full-scale voltage range is proportional to the resultof the following mathematical expression:V_(EXC) X SENS×(P_(MAX)−P_(MIN))

Preferably said output includes a circuit configured to scale andcondition said output signal such that the differential voltage of saidoutput signal is equal to the result of the following mathematicalexpression:V_(EXC)×SENS×P

Preferably said circuit includes a voltage divider, a differentialamplifier, or a combination of the two.

Preferably said processor(s) is further configured to increase thenumber of digital values for said output signal by interpolation so asto improve the smoothness of the signal that is input to an IBP monitor.

Preferably said interpolation includes linear interpolation, nonlinearinterpolation, or a combination of the two.

In a seventh aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to determine the output differential voltage forsaid output signal; and having at least two terminals configured toprovide said output signal in a form suitable for input to an IBPmonitor;

wherein said second input, said processor(s) and said output areconfigured such that the voltage level of each of said two terminals issubstantially similar to that produced by an IBP transducer for the samepressure variations in a subject.

Preferably said interface configured such that configured such that theoutput signal at said terminals is obtained by adding the midpointvoltage of said excitation signal with respect to the electrical groundof said interface, to the midpoint of said output differential voltage,this output differential voltage being the result of the followingmathematical expression:V_(EXC)×SENS×P

Preferably said output includes a summing amplifier to add the midpointvoltages.

Preferably said interface configured such that the output signal at saidterminals is obtained by adding the midpoint voltage of said excitationsignal with respect to the electrical ground of said interface, to thevoltage at either terminal for said output differential voltage, thisoutput differential voltage being the result of the followingmathematical expression:V_(EXC)×SENS×P

Preferably said output is configured such that said midpoint voltage ofsaid excitation signal is added to the voltage at either terminal forsaid output differential voltage by a summing amplifier.

In an eighth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor;

wherein said second input is configured to provide an input impedancethat is within a predetermined range.

Preferably said predetermined range is greater than 200 ohms.

Preferably said processor(s) and said second input are configured suchthat a user can select the input impedance.

Preferably said second input includes one or more resistors between theinput terminals corresponding to said input impedance.

In a ninth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor;

wherein said output means is configured to provide an output impedancethat is within a predetermined range.

Preferably said predetermined range is smaller than 3,000 ohms.

Preferably said processor(s) and said output are configured such that auser can select the output impedance.

Preferably said output includes one or more resistors placed across theoutput terminals corresponding to said output impedance.

In a tenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions;

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and

a calibration device configured to provide a calibration signal to saidprocessor(s);

wherein said processor(s) is configured to calibrate said measurementsignal according to predetermined instructions.

Preferably calibration of said measurement signal can be initiated andaborted by a user.

Preferably calibration of said measurement signal is automaticallyinitiated and aborted by said processor(s) according to predeterminedinstructions.

Preferably calibration of said measurement signal is automaticallyinitiated at predetermined intervals, at intervals that depend ondeviations of said measurement signal from physiologically realisticsignals, or at a combination of both groups of intervals.

In an eleventh aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor;

at least one processor(s) configured to receive said measurement signaland said excitation signal and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor;

wherein said processor(s) is further configured to supply a zero signalto said output according to predetermined instructions.

Preferably said processor(s) is configured to supply said zero signal tosaid output when calibration of said measurement signal is in progressor when said measurement signal is distorted as a result of thecalibration.

Preferably said processor(s) is configured to supply either said zerosignal or said output signal to said output at any one time.

Preferably the sending of said zero signal and said output signal can beinitiated and aborted by a user.

Preferably the sending of said zero signal and said output signal can bealternated by a user.

Preferably said zero signal is supplied to said output upon power-up ofsaid interface.

Preferably said zero signal is always supplied to said output wheneverno other signal is supplied to said output.

Preferably the output signal for said zero signal, is within ±75 mmHg.

In a twelfth aspect the present invention consists in a method ofzeroing an interface configured to connect between a noninvasive bloodpressure (NIBP) sensor and an invasive blood pressure (IBP) monitorcomprising the steps of

preparing said IBP monitor to receive said zero signal through anoutput, according to operating instructions for said IBP monitor;

sending said zero signal to said IBP monitor through said output;

zeroing on said IBP monitor, according to operating instructions forsaid IBP monitor; and

preparing said IBP monitor to receive said NIBP measurement signalthrough said output, according to operating instructions for said IBPmonitor.

In a thirteenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor; configured to determine the differentialvoltage of said excitation signal; and configured to determine themidpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signaland said differential voltage and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions;

an output configured to receive said midpoint voltage and configured toprovide said output signal in a form suitable for input to an IBPmonitor; and such that the midpoint of said output signal issubstantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the midpointof said output differential voltage.

In a fourteenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor; configured to determine the differentialvoltage of said excitation signal; and configured to determine themidpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signaland said differential voltage and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions;

an output configured to receive said midpoint voltage and configured toprovide said output signal in a form suitable for input to an IBPmonitor; and such that the midpoint of said output signal issubstantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the voltage ateither terminal for said output differential voltage.

In a fifteenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor; configured to determine the differentialvoltage of said excitation signal; and configured to determine themidpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signal,said differential voltage and said midpoint voltage and emulate anoutput signal indicative of the IBP of a subject according topredetermined instructions;

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and such that the midpoint of said outputsignal is substantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the midpointof said output differential voltage.

In a sixteenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive a transducer excitation signalprovided by an IBP monitor; configured to determine the differentialvoltage of said excitation signal; and configured to determine themidpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signal,said differential voltage and said midpoint voltage and emulate anoutput signal indicative of the IBP of a subject according topredetermined instructions;

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and such that the midpoint of said outputsignal is substantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the voltage ateither terminal for said output differential voltage.

In a seventeenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive the differential voltage of anexcitation signal provided by an IBP monitor; and configured todetermine the midpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signaland said differential voltage and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions; and

an output configured to receive said midpoint voltage and configured toprovide said output signal in a form suitable for input to an IBPmonitor; and such that the midpoint of said output signal issubstantially similar to that of said midpoint voltage.

In a eighteenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive the differential voltage of anexcitation signal provided by an IBP monitor; and configured todetermine the midpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signaland said differential voltage and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions;

an output configured to receive said midpoint voltage and configured toprovide said output signal in a form suitable for input to an IBPmonitor; and such that the midpoint of said output signal issubstantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the midpointof said output differential voltage.

In a nineteenth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive the differential voltage of anexcitation signal provided by an IBP monitor; and configured todetermine the midpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signaland said differential voltage and emulate an output signal indicative ofthe IBP of a subject according to predetermined instructions;

an output configured to receive said midpoint voltage and configured toprovide said output signal in a form suitable for input to an IBPmonitor; and such that the midpoint of said output signal issubstantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the voltage ateither terminal for said output differential voltage.

In a twentieth aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive the differential voltage of theexcitation signal provided by an IBP monitor; and configured todetermine the midpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signal,said differential voltage and said midpoint voltage and emulate anoutput signal indicative of the IBP of a subject according topredetermined instructions; and

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and such that the midpoint of said outputsignal is substantially similar to that of said midpoint voltage.

In a twenty-first aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive the differential voltage of theexcitation signal provided by an IBP monitor; and configured todetermine the midpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signal,and said differential voltage and said midpoint voltage and emulate anoutput signal indicative of the IBP of a subject according topredetermined instructions;

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and such that the midpoint of said outputsignal is substantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the midpointof said output differential voltage.

In a twenty-second aspect the present invention consists in an interfaceconfigured to connect between a noninvasive blood pressure (NIBP) sensorand an invasive blood pressure (IBP) monitor comprising

a first input configured to receive a measurement signal indicative ofthe NIBP of a subject;

a second input configured to receive the differential voltage of theexcitation signal provided by an IBP monitor; and configured todetermine the midpoint voltage of said excitation signal;

at least one processor(s) configured to receive said measurement signal,said differential voltage and said midpoint voltage and emulate anoutput signal indicative of the IBP of a subject according topredetermined instructions;

an output configured to provide said output signal in a form suitablefor input to an IBP monitor; and such that the midpoint of said outputsignal is substantially similar to that of said midpoint voltage;

said processor(s) and output are configured to provide an outputdifferential voltage in relation to said measurement signal in a formsuitable for input to an IBP monitor; and

said output is configured to add said midpoint voltage to the voltage ateither terminal for said output differential voltage.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

The invention consists in the foregoing and also envisages constructionsof 0 which the following gives examples.

BRIEF DESCRIPTION OF DRAWINGS

One preferred form of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a prior art invasive blood pressure(IBP) monitoring system.

FIG. 2 is a block diagram of a prior art noninvasive blood pressuremonitoring (NIBP) system comprising an NIBP sensor and a control anddisplay monitor.

FIG. 3 is a block diagram of a prior art NIBP monitoring systemcomprising an NIBP sensor, a control and display monitor, and aninterface.

FIG. 4 is an electrical schematic diagram of a prior art invasive bloodpressure transducer with a sensitivity of 5 μV/V/mmHg.

FIG. 5 is a block diagram of an NIBP monitoring system comprising anNIBP sensor, an interface, a calibrator, an adapter, an IBP interfacecable, and an IBP monitor.

FIG. 6 is a block diagram of an embodiment of the NIBP monitoring systemof FIG. 5 in which the interface uses a personal computer as thecontroller.

FIG. 7 is a block diagram of an embodiment of the NIBP monitoring systemof FIG. 5 in which the interface uses a microcontroller as thecontroller.

FIG. 8 is a block diagram of an embodiment of the NIBP monitoring systemof FIG. 5 in which the interface works with both a personal computer anda microcontroller to accomplish the essential functions and more.

FIG. 9 is a block diagram for the NIBP monitoring systems of FIGS. 6, 7and 8 showing the main elements of the interface and the general flow ofsignals.

FIG. 10 is a flowchart illustrating the general operation of aninterface designed in accordance with the present invention.

FIG. 11 is a block diagram of a proposed circuitry for interfacing withthe IBP monitor.

FIG. 12 is a flowchart illustrating an operating procedure for aninterface designed in accordance with the present invention.

FIG. 13 is a continuation of the flowchart of FIG. 12.

FIG. 14 is an illustration of the input, intermediate and output signalsof an interface designed in accordance with the present invention

DETAILED DESCRIPTION OF THE INVENTION

An intra-arterial pressure or IBP monitoring system typically comprisesa fluid-filled monitoring line (A-line) and an IBP monitor 100, asalready depicted in FIG. 1. The fluid-filled monitoring line typicallycomprises a catheter 105, tubing 110, an IBP transducer 115 (alsoreferred to simply as blood pressure transducer), an intravenous (IV)bag 120, and a pressure infusion cuff 125 for the IV bag 120. Most IBPtransducers in use are usually either disposable or semi-disposable. Asemi disposable transducer typically comprises a disposable dome thatcomes in contact with fluid in the system, and a reusable transducerthat does not come in contact with the fluid. An electrical schematic ofan IBP transducer is presented in FIG. 4.

An IBP interface cable is used to connect the transducer to the IBPmonitor. The IBP monitor can come in the form of a standalone monitor,as part of an integrated multiparameter patient monitor, or as a plug-inmodule or cartridge of a modular multiparameter monitor. The presentinvention relates to the replacement of the fluid-filled monitoring lineof an existing intra-arterial pressure monitoring system with anoninvasive system that is designed to work with the existing IBPmonitor. The noninvasive system comprises an NIBP sensor, an interface,and if required, a calibrator to calibrate the NIBP measurement signal.The interface connects between the NIBP sensor and the IBP monitor.

The preferred embodiment of the present invention comprises a singlemicroprocessor-based interface that connects a noninvasive bloodpressure (NIBP) sensor to an invasive blood pressure (IBP) monitor ormodule. The interface effectively emulates an IBP transducer in such away that the IBP monitor sees the interface as if it were a regular IBPtransducer from a fluid-filled pressure monitoring line. It converts theblood pressure measured using an NIBP sensor into an equivalent IBPtransducer output signal for input to an IBP monitor. Zeroing of theinterface with the IBP monitor can be easily performed in a way that issimilar to that for a fluid-filled system. A noninvasive systemcomprising a suitable NIBP sensor and this interface can be used as analternative to the fluid-filled monitoring line.

The interface according to the present invention operates based on aknown IBP transducer sensitivity, accepts the excitation voltageprovided by the IBP monitor, and produces an equivalent IBP transduceroutput signal corresponding to the measured blood pressure. Theinterface also emulates the input and output impedances of the IBPtransducer which the IBP monitor is configured to work with. The IBPmonitor itself may be connected to a central monitoring system, but thisconnection may not be essential.

This interface offers the advantage of enabling continuous beat-to-beatblood pressure to be monitored by noninvasive means, while allowing themedical staff to continue to use existing IBP monitors which they arealready familiar with. It allows medical staff to continue to benefitfrom multiparameter monitoring offered by patient monitors that providemonitoring of vitals signs such as ECG, oxygen saturation, respiration,and cardiac output, in addition to IBP. It also allows them to continueto benefit from the use of the central monitoring system to which theIBP or patient monitors are connected.

Characteristics of IBP Transducers Intended for Fluid-filled BloodPressure Monitoring

The sensor in most IBP transducers for fluid-filled monitoring consistsof four sensing elements 250 255 260 265 of the same nominal resistancearranged in a full-bridge circuit, as illustrated in FIG. 4. The bridgehas four terminals: E+ 270 and E− 275 for the input excitation voltage,and S+ 280 and S− 285 for the output signal. These four terminals areconnected to the IBP monitor through a transducer cable and an IBPinterface cable.

The two excitation voltage terminals receive the input excitationvoltage supplied by the IBP monitor, while the two output signalterminals present to the IBP monitor the output voltage representing theblood pressure being measured. The nominal midpoint voltage of theoutput signal is the same as the midpoint voltage between the excitationterminals, this condition being the result of the sensing elementshaving the same nominal resistance. In other words, the midpoint of thedifferential output voltage is offset with respect to the negativeexcitation terminal E− by half the voltage across the excitationterminals. For example, if the voltage of the positive terminal E+ 270with respect to the negative terminal E− is 5 V, the voltage across theoutput terminals S+ and S− will be such that the midpoint of this outputvoltage with respect to E− is 2.5 V.

Most IBP transducers in use conform to the ANSI/AAMI BP22—1994 standardfor blood pressure transducers, accepting an excitation voltage of 4 to8 VRM5 at a frequency of 0 to 5 kHz, and having a sensitivity of 5μV/N/mmHg (5 μV output per volt of excitation voltage per mmHg ofpressure), an input impedance greater than 200 ohms, an output impedancesmaller than 3,000 ohms, and a zero balance within ±75 mmHg. At leastone semi-disposable transducer, the HP 1290A by Hewlett Packard (laterAgilent Healthcare, now Philips Medical Systems, Andover, Mass.,U.S.A.), however, has a sensitivity of 40 μV/V/mmHg.

Emulation of IBP Transducers Intended for Fluid-filled Blood PressureMonitoring

The present invention relates to an interface that emulates an IBPtransducer in such a way that that the IBP monitor sees the interface asif it were a regular IBP transducer from a fluid-filled pressuremonitoring system. It senses the excitation voltage supplied by themonitor and for any given measured blood pressure, outputs an equivalentIBP transducer output signal to the IBP monitor, this signal being thesame as the signal that would have been produced for that pressure by atransducer which the IBP monitor is configured to work with. Forexample, for a transducer sensitivity of 5 μV/V/mmHg, an excitationvoltage of 5 V and a pressure of 100 mmHg, a IBP transducer will outputa differential voltage of (5 μV/V/mmHg)×(5 V)×(100 mmHg), or 2.5 mV. Forthe same combination of sensitivity, excitation voltage and pressure,the interface will also output the same differential voltage of 2.5 mV.This differential voltage can be expressed algebraicly:V_(EXC) X SENS×Pwhere V_(EXC) is the root-mean-square (RMS) differential voltage acrossthe excitation terminals, SENS is the transducer sensitivity which theIBP monitor is configured to work with, and P is the pressuremeasurement. The interface also emulates the input and output impedancesof the transducer.Single Interface Between NIBP Sensor and IBP Monitor

The present invention in one embodiment includes a singlemicroprocessor-based interface that connects between an NIBP sensor andan IBP monitor. A block diagram of an NIBP monitoring system thatcomprises an NIBP sensor 302, an interface 300, a calibrator 304, anadapter 306, an IBP interface cable 312, and an IBP monitor 318 ispresented in FIG. 5. The interface 300 connects to the IBP monitor 318through the adapter 306 followed by the IBP interface cable 312. Theadapter 306 and interface cable 312 each provides 4 wires to transmitthe 4 signals corresponding to the four terminals E+, E−, S+ and S− ofthe IBP transducer. One end of the adapter 306 has a connector 308 thatconnects to the interface 300 and the other end has a connector 310 thatconnects to the IBP interface cable 312. The adapter may or may notinclude a cable between its two connectors.

In practice, the calibrator 304 may or may not be required, depending onwhether the noninvasive method that is used for measuring blood pressurerequires the NIBP measurement signal to be calibrated against bloodpressure measurements made by another device. For example, the arterialtonometry method requires the NIBP measurement signal to be calibratedagainst blood pressure measurements made by another device, whereas thevascular unloading method does not. This is because the aerial tonometrymethod is not capable of establishing an absolute reference pressurelevel for its NIBP measurement signal, whereas the vascular unloadingmethod has built-in calibration capability.

IBP interface cables are usually transducer-specific at the transducerend and monitor-specific at the IBP monitor's end, because different IBPtransducers usually we different connector designs, as do differentmonitors. For example, an IBP interface cable that connects a BectonDickinson (BD) (Franklin Lakes, N.J., U.S.A.) disposable transducer to aHewlett-Packard (HP) IBP monitor (Philips Medical Systems) cannot beused to connect a Utah Medical Products (Midvale, Utah, U.S.A.)disposable transducer to the same monitor. The same IBP interface cablewill not connect to a Datex-Ohmeda (Madison, Wis., U.S.A.) monitor.Although most IBP transducers use proprietary connectors, thetelephone-type RJ11 plug (male part) is increasingly being used as astandard connector for disposable IBP transducers that are intended forfluid-filled, invasive blood pressure monitoring systems.

In order to connect the interface 300 to the IBP monitor 318, theinterface-cable end of the adapter 306 must be designed in such a waythat it can connect to the transducer end of the IBP interface cable312. The interface-cable end of the adapter could be designed to accepttake on the same connector as that on the cable of the transducer thatis used with the IBP interface cable. A hospital that uses a particularbrand of IBP transducers and a particular brand of IBP monitors wouldnormally already have IBP interface cables for connecting thetransducers to the monitors. The interface 300 can be designed toprovide for the use of transducer-specific adapters, in that for everyhospital that uses a different transducer, an adapter specific to thattransducer is provided. This practice will enable the interface 100 toconnect to existing IBP interface cables already in use, eliminating theneed for hospitals to purchase a different interface cable.

Digital Processing

There are at least two approaches to the interface circuitry: thedigital approach, and the analog approach. The digital approach uses acontroller to perform, through software or firmware, the device control,data acquisition, digital signal processing and data processing, leavingthe analog circuitry to perform signal conditioning. As a result ofthis, minimal analog circuitry is required, so that the cumulativeanalog signal level uncertainty caused by temperature changes isreduced. Although some uncertainty is introduced in the conversion ofanalog signal to digital signal and digital signal back to analogsignal, this uncertainty is likely to be significantly smaller than thatintroduced by largely analog circuitry. One major advantage of thedigital approach is that it offers the flexibility of allowing changesto device control, data acquisition, and signal and data processing tobe easily implemented through software or firmware.

The analog approach uses largely analog circuitry to perform thenecessary functions. Because of this, the analog circuitry is likely tobe much more complex, so that the cumulative analog signal leveluncertainty due to temperature changes is likely to be significantlygreater than if the digital approach is used. Although the analogcircuitry may not experience much error associated withanalog-to-digital or digital-analog conversion, this advantage is likelyto be overshadowed by the large signal uncertainly in the analogcircuitry.

The present invention in the preferred embodiment uses the digitalapproach. At least three different design configurations are possiblewith the digital approach. The first configuration, as illustrated inFIG. 6, uses a personal computer (PC) 355 as the only controller. Inthis configuration, the PC 355 forms an essential part of the design andthe interface 350 itself does not contain any controller. The PC 355 isused to perform, through software, extensive signal and data processing,both in real time and offline, as well as store the original andprocessed data. Although FIG. 6 rows that the calibrator 304 connectsdirectly to the interface 350, the interface 350 an be designed to havethe calibrator 304 connect directly to the PC 355 instead.

The second configuration, as illustrated in FIG. 7, uses amicrocontroller as the only controller. The microcontroller forms thecentral part of an embedded system. through firmware, it performs thesame functions as those for the PC but in a limited way because of itslower processing power compared to a PC. The calibrator 304 connects tothe interface 400.

The third configuration, as illustrated in FIG. 8, uses amicrocontroller as well as a PC 355. The microcontroller and PC 355 worktogether to accomplish the essential functions and more. Depending onthe hardware, software and firmware configurations, there are many waysin which the microcontroller and PC 355 can work together to accomplishthese functions. For example, the microcontroller and PC 355 could workindependently in such a way that the microcontroller performs the samefunctions as in the second design configuration, while the PC 355performs extensive signal and data processing to provide moreinformation about the blood pressure measurement. In this example, thePC 355 is not an essential part of the design in that, without the PC355, the interface 400 can still work with the IBP monitor 318. Asanother example, the microcontroller could perform the acquisition ofthe NIBP measurement signal and sending of digital values to thedigital-to-analog converter (DAC), while the PC 355 works in between toperform extensive signal and data processing. In this configuration,both the microcontroller and PC 355 are essential parts of the design.Whether the calibrator 304 is connected to the interface 400 or to thePC 355 depends on the functions performed by the microcontroller and thePC 355. This configuration also allows the use of features that comewith the PC 355, such as memory and data storage.

The interface can be designed to make all of above three configurationsavailable and to allow the user to select any one of them through acontrol panel on the interface. In general, other user input, such asuser-initiated calibration requests, can be effected through a controlpanel on the interface or though the PC.

General Operation of Interface

A block diagram of an NIBP monitoring system showing the main elementsof an interface that uses the digital approach is presented in FIG. 9.The general operation of the interface involves the following steps,which are also illustrated in FIG. 10:

-   a) Step 551. The controller 500 reads the excitation voltage    supplied by the IBP monitor 318, through an excitation signal    conditioner 506 and an analog-to-digital converter (ADC) 508.-   b) Step 552. The controller 500 receives the NIBP measurement signal    from the NIBP sensor 302 through a sensor signal conditioner 502 and    an ADC 504.-   c) Step 553. The controller 500 processes the NIBP measurement    signal.-   d) Step 554. The controller 500 calibrates the processed NIBP    measurement signal if necessary.-   e) Step 555. The controller 500 determines the measured blood    pressure in mmHg.-   f) Step 556. The controller computes the equivalent differential IBP    transducer output voltage for the measured blood pressure as the    product of the transducer sensitivity, excitation voltage and    measured blood pressure.-   g) Step 557. The controller 500 computes an appropriate digital    value that is proportional to this equivalent differential voltage.-   h) Step 558. The controller 500 sends the digital value to a DAC    510.-   i) Step 559. The output signal conditioner 512 scales and conditions    the output voltage of the DAC 510 and outputs the equivalent IBP    transducer output voltage for input to the IBP monitor 318.    Transducer Sensitivity

In order for the interface to be able to output the correct equivalentIBP transducer output signal to the IBP monitor for any measured bloodpressure, the controller needs to know in advance the transducersensitivity which the IBP monitor is configured to work with, as well asthe excitation voltage supplied by the monitor. The excitation voltageis supplied by the IBP monitor and sensed by the controller through theexcitation signal conditioner. However, the transducer sensitivity needsto be provided to the interface by the user, and it must be the same asthat of the transducer sensitivity which the IBP monitor is configuredto work with. This sensitivity is normally specified in the monitor'smanual.

The interface can be designed in such a way that the transducersensitivity is selectable by the user from among two or moresensitivities. Additionally, it can be designed to operate on a defaultsensitivity of 5 μV/V/mmHg, the most commonly used sensitivity, if noselection is made.

Digital Output for Measured Blood Pressure

For the same pressure measurement range and the same excitation voltage,a larger transducer sensitivity will give a larger range of outputvoltage. For example, for a pressure measurement range of 0 to 300 mmHgand an excitation voltage of 5 V, a 40 μV/V/mmHg transducer will give afull output voltage range of 60 mV (40 μVN/mmHg×5 V×300 mmHg), whereas a5 μV/V/mmHg transducer will give an output voltage range of only 7.5 mV.If the DAC output voltage is to be scaled down by a factor of 100, theDAC must be able to output a voltage range of at least 6 V for the 40μV/V/mmHg transducer and 0.75 V for the 5 μVN/mmHg transducer. Thisoutput voltage range can be expressed algebraicly:V_(EXC) X SENS×(P_(MAX)−P_(MIN))where V_(EXC) is the root-mean-square (RMS) differential voltage acrossthe excitation terminals, SENS is the transducer sensitivity which saidIBP monitor is configured to work with, and PMAX and PMIN arerespectively the maximum and minimum pressures which said interface isconfigured to work with.

If a DAC whose resolution in LSBN (least significant bits per volt) hasbeen maximized for a 40 μVN/mmHg transducer sensitivity is used for a 5μVN/mmHg sensitivity, the resolution of the DAC output voltage and hencethe number of data points will be reduced, so that the blood pressurewaveform becomes more steplike or less smooth than that obtained with aDAC whose resolution has been optimized for the 5 μVN/mmHg sensitivity.The more steplike a waveform is, the less it represents the actualwaveform.

To improve the smoothness of the blood pressure waveform at the IBPmonitor's end, the resolution of the DAC output voltage in LSBN shouldbe maximized in such a way that the DAC is still able to produce therequired DAC output voltage range, which, as indicated above, depends onthe transducer sensitivity, excitation voltage, pressure measurementrange, and the scaling factor for the DAC output voltage. All this canbe accomplished by using a programmable DAC that allows its full-scaleoutput voltage range to be configured by the controller, and byconfiguring the DAC for a full-scale output voltage range that isslightly larger than the required DAC output voltage range.

To further improve the smoothness of the blood pressure waveform that isdisplayed on the IBP monitor, the number of the digital values for thewaveform can be increased by interpolation. A simple method is toperform linear interpolation between every two adjacent data points.Nonlinear interpolation methods such as quadratic interpolation andcubic spline interpolation can also be used.

Emulation of IBP Transducer Output Signal

For a full emulation of the output voltage level of the IBP transducer,the equivalent IBP transducer output voltage produced by the interfaceshould be such that the voltage level of each of the two terminals forthis equivalent voltage is the same as the level that would be producedat the corresponding output terminal of the transducer. Since thenominal midpoint voltage of the IBP transducer output signal is the sameas the midpoint voltage between the excitation terminals, as mentionedabove, this emulation can be accomplished by centering the differentialoutput voltage about the midpoint voltage between the excitationterminals. In other words, the midpoint of the differential outputvoltage rides on the midpoint voltage between the excitation terminals,or the midpoint of the differential output voltage is offset withrespect to the negative excitation terminal E− by half the voltageacross the excitation terminals.

An approximate emulation of the IBP transducer output voltage level canbe achieved by making one of the terminals for the differential outputvoltage take on the midpoint voltage between the excitation terminals.This approximate emulation is judged to be adequate because thedifferential output voltage is relatively small, being usually in theorder of millivolts or tens of millivolts, compared to the midpointvoltage between the excitation terminals, which is of usually in theorder of volts as measured with respect to the negative excitationterminal E−. Additionally, the circuitry for implementing thisapproximate emulation is likely to be simpler than that for the fullemulation.

It is possible that without emulating this offset, the output voltage ofthe interface will still be accepted by most IBP monitors. For example,the HP1006B IBP module (Philips Medical Systems) has been shown in thelaboratory to accept the output voltage from the interface when thenegative output terminal S− is connected to the electrical ground of theinterface. To be conservative, however, the voltage level of the outputterminals should be emulated in case the IBP monitor uses this voltagelevel, among other characteristics, to check for proper functioning ofthe IBP transducer.

Input and Output Impedances

It is known that IBP monitors use the input impedance, output impedance,or both to detect the presence and absence of a transducer or whetherthe transducer is functioning properly, so these impedances should beemulated. Emulating these impedances will more accurately emulate theactual situation and help to reduce the chances of problems incommunication between the interface and IBP monitor.

Circuitry for Interfacing with IBP Monitor

An electrical schematic diagram of a proposed circuitry for interfacingwith the IBP monitor is presented in FIG. 11. This circuitry providesfor interfacing with the excitation and input terminals of the IBPmonitor. It implements the approximate emulation discussed above. Thecircuitry includes unity-gain voltage followers 608 or buffers atvarious places to minimize loading and to ensure that any malfunctioningof the interface will not compromise the electrical safety andelectrical performance of the IBP monitor.

Referring to FIG. 11, a voltage divider consisting of four resistors R1600, R2 602, R3 604 and R4 606 of predetermined values and in series isplaced across the excitation terminals such that R1 600 equals R4 606and R2 602 equals R3 604. This means that the sum of R1 600 and R2 602is equal to the sum of R3 604 and R4 606, so that the connection pointbetween R2 602 and R3 604 gives the midpoint voltage 630 between theexcitation terminals. The voltage across R2 602 and R3 604 represents ascaled version of the voltage across the excitation terminals, thescaling factor being given by the ratio (R2+R3)/(R1+R2+R3+R4). Thisscaled voltage is supplied to the input of a bipolar differential-inputADC 610, and the output of the ADC 610 is read by the controller 500.The bipolar ADC 610 accepts positive and negative input differentialvoltages. From this ratio, the controller 500 derives the voltage acrossthe excitation terminals. The four resistors are selected in such a waythat their sum is within the input impedance range of the IBP transducerwhich the IBP monitor is configured to work with—this is to emulate theinput impedance of the transducer. Additionally, R2 602, R3 604 and theADC 610 are configured in such a way that the voltage across R2 602 andR3 604 is within the full-scale input voltage range of the ADC 610.

From the voltage across the excitation terminals, transducer sensitivityand measured blood pressure, the controller 500 generates an appropriatedigital value representing the measured blood pressure and sends thedigital value to a bipolar DAC 620. The bipolar DAC 620 provides forpositive and negative output voltages. The output voltage of the DAC 620is scaled by means of a differential amplifier in such a way that thatthe output voltage is the same as the differential voltage that wouldhave peen produced at the transducer output for the same transducersensitivity, the same voltage across the excitation terminals, and thesame measured blood pressure. This scaled differential voltage is thensuperimposed on the midpoint voltage 630 between the excitationterminals by means of a summing amplifier 624. The final output issupplied to the IBP monitor. A resistor R5 626 is placed across theoutput terminals such that R5 626 is within the output impedance rangeof the IBP transducer which the IBP monitor is configured to workwith—this is to emulate the output impedance of he the transducer.Additionally, the DAC 620 and scaling factor for the DAC output voltageare selected in such a way that the DAC full-scale output voltage rangeincludes the DAC output voltage range corresponding to the full range ofthe blood pressure which the interface is designed to measure.

Alternatives for various parts of the circuitry are available. First,the above circuitry uses the analog midpoint voltage 630 for the summingamplifier 624. One alternative method of obtaining the midpoint voltageis to have the controller read the midpoint excitation voltage through asignal conditioner and an ADC, generate a digital value and send thedigital value to a DAC, and have a signal conditioner condition the DACoutput voltage to the same level as that of the actual midpoint voltage.Another alternative method is to have the controller read the voltagesat the excitation terminals E+ 270 and E− 275 and compute the midpointexcitation voltage, instead of reading the midpoint voltage directly. Inboth alternative methods, the circuitry can be designed to use an ADCwith a fixed full-scale input voltage range, or me whose full-scaleinput voltage range can be configured through software or firmware insuch a way that the measurement resolution in V/LSB is maximized to givea more accurate measurement of the excitation voltage.

Second, the voltage across the excitation terminals in the abovecircuitry is sensed through a voltage divider. This voltage can also besensed through a differential amplifier or a combination of a voltagedivider and a differential amplifier.

Third, the above circuitry uses a differential-input ADC to receive thescaled voltage across the excitation terminals and the controller tocompute the actual differential voltage based on the scaling factor. Onealternative method of obtaining the actual differential voltage is touse a single-ended-input ADC to receive the actual voltage of each ofthe excitation terminals E+ 270 and E− 275 or a scaled version of it,and have the controller compute the actual differential voltage.

Fourth, the midpoint voltage 630 between the excitation terminals in theabove circuitry is sensed through a voltage divider. This midpointvoltage can also be sensed through a combination of a voltage dividerand a differential amplifier.

Fifth, the above circuitry uses a differential amplifier 622 to scalethe DAC output voltage. An alternative method to scale the DAC output isto use a voltage divider or a combination of a voltage divider and adifferential amplifier.

It is recognized that the scaling factor for the DAC output voltage willdiffer from circuit assembly to circuit assembly because of variationsin the components and at these variations will ultimately affect theaccuracy of the voltage across the output terminals S+ 280 and S− 285.One way to resolve this problem is to determine the actual DAC outputvoltage scaling factor for every interface unit and then use the valuefor computational purposes in the software or firmware. Each interfaceunit will then have its own DAC output voltage scaling factor. This willhelp ensure that the correct output voltage s produced across the outputterminals S+ 280 and S− 285.

It should be noted that the negative excitation terminal E− 275 shouldnot be connected to the electrical ground of the interface, because E−275 in general does not share the same electric potential as theelectrical ground of the interface. Connecting them can lead to a groundloop that can damage both the interface and the IBP monitor,compromising the electrical safety and electrical performance of thedevices. For comparison, when an IBP transducer is used with an IBPmonitor, the E− terminal 275 of the transducer takes on whatever voltageis applied to that terminal by the IBP monitor 318. Using E− 275 as itis, that is, by not connecting it to the electrical ground of theinterface, emulates this situation.

Calibration of NIBP Measurement Signal

For an NIBP monitor, calibration establishes the absolute referenceblood pressure corresponding to the NIBP measurement signal. For an NIBPmonitor that does not have built-in calibration capability, its NIBPmeasurement signal must be calibrated against blood pressuremeasurements made by another device. One such NIBP monitor is describedin U.S. Pat. No. 6,443,906 entitled METHOD AND DEVICE FOR MONITORINGBLOOD PRESSURE, the contents of which are incorporated herein byreference. An embodiment of this monitor comprises a tonometric NIBPsensor, a watch head that houses a microprocessor, and a strap. In thisembodiment, the monitor is strapped to the wrist, with the NIBP sensorbeing placed over the radial artery to detect blood pressure in theartery. Wrist movement and changes in the properties of compressedtissue between the monitor and the wrist can cause the sensor todisplace from its original position and the strap tension to change.This displacement and tension change will change the forces acting onthe contact area between the sensor and the wrist. This change in forceswill alter the NIBP measurement signal even if there has been no changein the arterial pressure.

For an NIBP monitoring system that uses the interface and the abovetonometric NIBP sensor, calibration should be performed at the beginningof an NIBP measurement period and whenever there is reason to suspectthat the alteration of the NIBP measurement signal might not have beencaused by a change in the arterial pressure. The interface can bedesigned to activate a calibrator whenever calibration is required. Forexample, the interface can be designed to allow the user to initiate andabort a calibration, either by pressing a button on the interface orclicking on a button on the computer screen. It can also be designed toautomatically initiate a calibration at predetermined intervals, atintervals that depend on deviations of the NIBP measurement signal fromphysiologically realistic signals, or at a combination of both groups ofintervals, and to automatically abort the calibration when necessary.

It should be noted that the choice of a calibrator for an NIBP monitorthat provides continuous beat-to-beat blood pressure measurement is notrestricted to those that use an occlusive cuff. Any blood pressuremeasurement device that is capable of providing accurate blood pressuremeasurement can potentially be used as a calibrator.

Zeroing of IBP Transducer with IBP Monitor

The output voltage of an IBP transducer at zero mmHg is usually notzero. This output voltage is called the zero offset or zero balance.This offset voltage is sometimes augmented by hydrostatic pressurecaused by a column of fluid above the level of the sensing area of thetransducer. For accurate IBP measurement, the IBP transducer must bezeroed with the IBP monitor before monitoring begins. During thezeroing, the IBP, monitor effectively reads the total offset voltage andassociates it with zero mmHg, or strictly speaking, zero gage pressure,and in doing so, establishes a zero-mmHg reference level for the IBPmonitor.

The zeroing procedure for a fluid-filled pressure monitoring systemrequires the clinician to manually trigger the IBP monitor to performthe zeroing. It includes the following steps:

-   a) Prepare the IBP monitor to receive the transducer output voltage    at zero mmHg.-   b) Position the zeroing port of the IBP transducer so that it is at    the patient's mid-heart level.-   c) Turn the handle of the zeroing stopcock OFF to the patient and    loosen or remove the deadender cap on the zeroing side port. This    step blocks the transducer from the patient's intra-arterial    pressure and opens the transducer to the atmosphere. Some fluid will    flow out of the side port as a result.-   d) Zero the transducer with the IBP monitor by pressing the    appropriate key or button on the IBP monitor. This zeroing has to be    activated manually because there is no automated feedback to check    whether or not the fluid-filled system is ready to be zeroed.-   e) Turn the stopcock handle OFF to the zeroing side port (closing    the port to atmosphere) to re-admit patient's pressure. The    patient's intra-arterial pressure waveform will now show up on the    IBP monitor.-   f) Tighten the deadender cap to close the side port.    Zeroing of Interface with IBP Monitor

For an NIBP monitoring system that uses the interface, the tonometricNIBP sensor as mentioned above, and a calibrator that uses an occlusivecuff, the initial zeroing procedure could include the following steps:

-   a) Prepare the IBP monitor to receive the transducer output voltage    at zero mmHg.-   b) Calibrate the NIBP measurement signal using the calibrator, with    the applied cuff at the patient's mid-heart level. From the    reference blood pressures measured by the calibrator and the NIBP    measurement signal received, the controller will establish a    calibration relationship that relates the NIBP measurement signal to    blood pressure in mmHg. Because an IBP transducer in general does    not require any calibration when it is in use, this step can be    considered to be only partially equivalent to positioning the    zeroing port of the BP transducer at the patient's mid-heart level,    as described in step (b) of the transducer zeroing procedure.-   c) Send a zero-mmHg signal to the IBP monitor, by sending to the DAC    a digital value corresponding to zero mmHg. The sending of this    signal can be initiated automatically by the interface, or by the    user pressing a button on the interface or on the computer screen.    This step is equivalent to opening the IBP transducer to the    atmosphere, as described in step (c) of the transducer zeroing    procedure. This digital value can be a zero or a non-zero value, as    long as the zero-mmHg signal that is sent to the IBP monitor is    within the range corresponding to the range of the zero balance for    which the IBP monitor is configured to work with. Although the AAMI    standard stipulates a zero balance within ±75 mmHg, as mentioned    above, most IBP monitors are designed to accept a larger range of    zero balance.-   d) Zero the interface with the IBP monitor, by pressing the    appropriate key or button on the IBP monitor. This step is    equivalent to zeroing the IBP transducer, as described in step (d)    of the transducer zeroing procedure.-   e) Send equivalent IBP transducer output voltage to the IBP monitor    by pressing a button on the interface or clicking on a button on the    computer screen. The interface will send the equivalent IBP    transducer output voltage to the IBP monitor. This step is    equivalent to turning the stopcock handle of the BP transducer OFF    to the zeroing side port to re-admit the patient's pressure, as    described in step (e) of the transducer zeroing procedure.

It should be noted that the above NIBP monitoring system has noequivalent step for step (f) of the transducer zeroing procedure.

The above procedure indicates that zeroing is performed and calibration.Calibration can actually be performed before the zeroing, meaning theabove steps for the NIBP monitoring system can proceed in the order ofa, c, d, b, and e. Any subsequent re-calibration can proceed without anyzeroing unless it is required for other reasons. If no calibration isrequired, step (b) can be removed. In any case, zeroing and calibrationshould be performed only after sufficient time has been given for theNIBP monitoring system to warm up after its initial power-up.

If a re-calibration is performed while the equivalent IBP transduceroutput signal is still being sent to the IBP monitor, the blood pressurewaveform will appear distorted if the occlusive cuff of the calibratoris applied to the same arm as the NIBP sensor. This is because theocclusion of the brachial artery by the cuff will alter the normaltransmission of arterial pressure to the radial artery. Although thedistorted waveform can actually be used to indicate that calibration isin progress, it may inadvertently confuse the clinician. The software orfirmware of the NIBP monitoring system can be designed to provide theoption of temporarily not displaying the distorted waveform on the IBPmonitor whenever calibration is in progress. This can be achieved bysending a zero-mmHg signal to the IBP monitor to display a zero-mmHgline and at the same time displaying a “calibration in progress” messageon the monitor to inform the clinician.

If the calibrator cuff is applied to the opposite arm, the bloodpressure waveform will not be distorted during calibration, so it willnot be necessary to prevent the waveform from being displayed on the IBPmonitor unless there is reason to do so. This means that the waveformcan continue to be displayed on the IBP monitor whether or notcalibration is in progress. However, because the clinician may apply theocclusive cuff to the same arm or a different arm, it may be advisableto design the interface in such a way that a zero-mmHg signal is alwayssent to the IBP monitor when calibration is in progress.

Depending on the stability of the interface output signal at zero mmHg,it may be necessary to perform zeroing at regular or preprogrammedintervals. The zeroing can be followed, though not necessarily, by acalibration. For example, if the ambient temperature fluctuates too muchafter the NIBP monitoring system has been zeroed and calibrated for thefirst time at the start of a measurement period, a re-calibrationfollowed by a re-zeroing is advisable. In general, however, acalibration of the NIBP monitoring system, whether initiated by the useror automatically by the system, need not always be followed by a zeroingunless there is reason to do so.

To ensure that no unwanted signal is displayed on the IBP monitor, theinterface can be configured in such a way that by default, a zero-mmHgsignal is always automatically sent to the IBP monitor whenever theinterface is powered up. Additionally, the buttons that are used by theuser to send a zero-mmHg signal and the equivalent IBP transducer outputvoltage to the IBP monitor can be designed to be the same button thattoggles between the two functions.

Operating Procedure for Interface

A flowchart for an operating procedure that includes calibration andzeroing is illustrated in FIGS. 12 and 13. It involves the following:

-   a) Upon power-up of the interface in step 650, a zero-mmhg signal is    sent by the interface to the IBP monitor through step 652.-   b) The excitation voltage is read through step 654.-   c) The NIBP measurement signal is received through step 656 and    processed through step 658.-   d) If calibration has not been performed before or if it is required    for other reasons, it is performed through step 664, after which the    controller returns to step 656 to continue receiving the NIBP    measurement signal.-   e) If zeroing has not been performed before or if it is required for    other reasons, it is performed through steps 670, 672, 674 and 676,    after which the controller returns to step 656 to continue receiving    the NIBP measurement signal.-   f) If calibration and zeroing is not required, the measured blood    pressure is determined through step 678.-   g) The equivalent differential IBP transducer output voltage for the    measured blood pressure is computed through step 680.-   h) The appropriate digital value for this differential voltage is    computed through step 682.-   i) The computed digital value is send to the DAC through step 684.-   j) If continuation of the NIBP measurement is required, the    controller returns to step 656 to continue receiving the NIBP    measurement signal.-   k) If continuation of the NIBP measurement is not required, the    procedure ends.    Input, Intermediate and Output Signals

An illustration of the input, intermediate and output signals for aninterface using the tonometric NIBP sensor mentioned above and atransducer sensitivity of 5 μV/V/mmHg is presented in FIG. 14. The NIBPmeasurement signal 700 and excitation voltage 705 are the input signals.The calibrated NIBP measurement signal 710 is the intermediate signal,which is generated by the controller. The equivalent differential IBPtransducer output voltage 715 is the same as the voltage across theoutput terminals S+ 280 and S− 285 of the interface (FIG. 11).

FIG. 14 shows a calibrated signal with a systolic pressure of 120 mmHgand a diastolic pressure of 80 mmHg for the first beat of the waveform.The corresponding equivalent differential IBP transducer output voltagefor the systolic pressure is given by (120 mmHg×5 V×5 μV/V/mmHg), or3,000 μV. Similarly, the corresponding equivalent differential IBPtransducer output voltage for the diastolic pressure is given by (80mmHg×5 V×5 μV/V/mmHg), or 2,000 μV. These output voltages are indicatedin the output signal block 715.

1. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input confiqured to receive a measurement signal indicative of the NIBP of a subject; a second input confiqured to receive a transducer excitation signal provided by the IBP monitor; at least one processor(s) confiqured to receive said measurement signal and said excitation signal and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; and an output configured to provide said output signal in a form suitable for input to the IBP monitor; wherein said first input is connected to the NIBP sensor and said second input and said output are both connected to the IBP monitor; and wherein said second input and said output are configured to connect to the IBP monitor by means of a detachable or configurable connection between said interface and an IBP interface cable for the IBP monitor.
 2. An interface as claimed in claim 1 wherein said connection includes an adapter with a first connector configured to connect to said interface and a second connector configured to connect to a transducer end of the IBP interface cable.
 3. An interface as claimed in claim 2 wherein said adapter includes a cable between said first and second connectors.
 4. An interface as claimed in claim 2 wherein said adapter does not include a cable between said first and second connectors.
 5. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive a transducer excitation signal provided by the IBP monitor; at least one processor(s) configured to receive said measurement signal and said excitation signal and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; and an output configured to provide said output signal in a form suitable for input to the IBP monitor; wherein said processor(s) is configured such that a user can select the sensitivity of said output signal in relation to said measurement signal from a predetermined range of choices.
 6. An interface as claimed in claim 5 wherein said sensitivity can be selected or specified by a user.
 7. An interface as claimed in claim 6 wherein said range includes 5 μV/V/mmHg and 40 μV/V/mmHg.
 8. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive a transducer excitation signal provided by the IBP monitor; at least one processor(s) configured to receive said measurement signal and said excitation signal and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; and an output configured to provide said output signal in a form suitable for input to the IBP monitor; wherein said output is configured to provide an output impedance that is within a predetermined range.
 9. An interface as claimed in claim 8 wherein said predetermined range is smaller than 3,000 ohms.
 10. An interface as claimed in claim 8 wherein said processor(s) and said output are configured such that a user can select the output impedance.
 11. An interface as claimed in claim 8 wherein said output includes one or more resistors placed across the output terminals corresponding to said output impedance.
 12. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive a transducer excitation signal provided by the IBP monitor; configured to determine the differential voltage of said excitation signal; and configured to determine the midpoint voltage of said excitation signal; at least one processor(s) configured to receive said measurement signal, said differential voltage and said midpoint voltage and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; an output configured to provide said output signal in a form suitable for input to the IBP monitor; and such that the midpoint of said output signal is substantially similar to that of said midpoint voltage; said processor(s) and output are configured to provide an output differential voltage in relation to said measurement signal in a form suitable for input to the IBP monitor; and said output is configured to add said midpoint voltage to the midpoint of said output differential voltage.
 13. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive a transducer excitation signal provided by the IBP monitor; configured to determine the differential voltage of said excitation signal; and configured to determine the midpoint voltage of said excitation signal; at least one processor(s) configured to receive said measurement signal, said differential voltage and said midpoint voltage and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; an output configured to provide said output signal in a form suitable for input to the IBP monitor; and such that the midpoint of said output signal is substantially similar to that of said midpoint voltage; said processor(s) and output are configured to provide an output differential voltage in relation to said measurement signal in a form suitable for input to the IBP monitor; and said output is configured to add said midpoint voltage to the voltage at either terminal for said output differential voltage.
 14. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive the differential voltage of an excitation signal provided by the IBP monitor; and configured to determine the midpoint voltage of said excitation signal; at least one processor(s) configured to receive said measurement signal, said differential voltage and said midpoint voltage and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; and an output configured to provide said output signal in a form suitable for input to the IBP monitor; and such that the midpoint of said output signal is substantially similar to that of said midpoint voltage.
 15. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive the differential voltage of an excitation signal provided by the IBP monitor; and configured to determine the midpoint voltage of said excitation signal; at least one processor(s) configured to receive said measurement signal, and said differential voltage and said midpoint voltage and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; an output configured to provide said output signal in a form suitable for input to the IBP monitor; and such that the midpoint of said output signal is substantially similar to that of said midpoint voltage; said processor(s) and output are configured to provide an output differential voltage in relation to said measurement signal in a form suitable for input to the IBP monitor; and said output is configured to add said midpoint voltage to the midpoint of said output differential voltage.
 16. An interface configured to connect between a noninvasive blood pressure (NIBP) sensor and an invasive blood pressure (IBP) monitor comprising: a first input configured to receive a measurement signal indicative of the NIBP of a subject; a second input configured to receive the differential voltage of an excitation signal provided by the IBP monitor; and configured to determine the midpoint voltage of said excitation signal; at least one processor(s) configured to receive said measurement signal, said differential voltage and said midpoint voltage and emulate an output signal indicative of the IBP of a subject according to predetermined instructions; an output configured to provide said output signal in a form suitable for input to the IBP monitor; and such that the midpoint of said output signal is substantially similar to that of said midpoint voltage; said processor(s) and output are configured to provide an output differential voltage in relation to said measurement signal in a form suitable for input to the IBP monitor; and said output is configured to add said midpoint voltage to the voltage at either terminal for said output differential voltage. 